Pneumotonometric IOP trace during advance of probe assembly. The full time course is presented in (
A), and the relationship between the cumulative probe advance and apparent IOP is provided in (
B). (
A) At time 0, the probe assembly was advanced from the air into the tear fluid. A characteristic slight initial decrease in pressure was noted, probably reflecting capillary action by the tear film. After contact of the probe diaphragm with the cornea, the pressure reading was 0 mm Hg. Then, the probe assembly was advanced by 1 mm (
first upward arrow). The pneumotonometric reading showed an abrupt rise and then quickly decayed to 14 to 15 mm Hg. Over the subsequent 7 minutes, the probe assembly was either advanced (
upward arrows) or retracted (
downward arrows) in 0.34- or 1-mm steps without producing any significant change in IOP reading. Because of the shift of the needle in the mount, the large advances of the full probe assembly were associated with very much smaller displacements of the probe tip membrane. Advancing the probe assembly beyond the supporting needle’s range of free movement (
Fig. 1B , position B) produced a sharp increase in the reading with only partial subsequent decay (
upward arrow at ∼520 seconds

). Then, after partially retracting the probe assembly by 0.34 mm (
downward arrow at ∼760 seconds), the IOP reading returned to approximately the same value as that observed over the range of positions examined earlier (at ∼100–520 seconds). At this point, advance of the probe assembly by even 0.05 mm again increased the IOP reading (
upward arrow at ∼840 seconds) and retraction at the subsequent
downward arrow (by 0.34 mm at ∼900 seconds) restored the reading to ∼14 mm Hg, consistent with the needle still being at end of its excursion range. Further retraction of the probe to permit free excursion of the needle but with persistent cornea-diaphragm contact restored a stable pressure plateau. Subsequent advances or retractions of the probe assembly did not affect the IOP reading during the remainder of the trace. (
B) Relationship between cumulative advance and measured pressure. A stable endpoint was observed over a wide range of advances of the probe assembly and was limited by the maximum range (∼4 mm) of free advance of the supporting needle within the inner cylinder of the probe mount. The pneumotonometric endpoint was taken to be ∼14 mm Hg, corresponding to the plateau in (
A), based on the relative insensitivity of IOP to movement of the probe assembly and large IOP pulsations synchronous with the cardiac pulse (see
Fig. 3 ).